Posterior Triangle Anatomy

• The learning objectives are to identify the boundaries, subdivisions, and contents of the posterior triangle, and to describe the attachments, nerve supply, and action of the Sternocleidomastoid muscle.

Posterior Triangle Boundaries

• Anterior: The posterior border of the sternomastoid muscle.
• Posterior: The anterior border of the trapezius muscle.
• Base: The intermediate 1/3 of the clavicle.
• Apex: Formed by the meeting of sternomastoid and trapezius muscles at the superior nuchal line.
• It is subdivided into two triangles by the omohyoid muscle. These triangles are the omoclavicular (also called the supraclavicular triangle) and the occipital triangle.

Posterior Triangle Roof

• Skin.
• Superficial fascia containing the platysma and external jugular vein.
• The investing layer of the deep cervical fascia.

Posterior Triangle Floor

• The floor is formed by muscles from below upwards including:
  • Scalenus medius.
  • Levator scapulae.
  • Splenius capitis.
  • A small part of semispinalis capitis.
• All muscles of the posterior triangle floor are covered by the prevertebral fascia.

Posterior Triangle Contents

• Nerves

  • 3 trunks of the brachial plexus.
  • 4 cutaneous branches of the cervical plexus:
    • lesser occipital
    • great auricular
    • transverse cervical
    • supraclavicular
  • Spinal accessory nerve (C1-C5 spinal segments).
    • It appears above the midpoint of the posterior border of the sternomastoid muscle.
    • It descends postero-inferiorly parallel to the levator scapulae and ends in the deep surface of the trapezius muscle. Injury to the spinal accessory nerve leads to paralysis of the mentioned muscles, turning the face towards the paralyzed side and affecting the normal sternomastoid muscle.
      • Lesser occipital nerve (C2) ascends upwards on the posterior border of sternomastoid to supply the posterior part of the scalp. - Great auricular nerve (C2, 3) crosses sternomastoid towards the angle of the mandible to supply the overlying skin. - Transverse cervical nerve (C2, 3) crosses sternomastoid towards the anterior triangle to supply the skin of the front of the neck. - The supraclavicular nerves (C3, 4) descend downwards and the anterior scalenus, as medial, intermediate and lateral groups to supply the skin on the front of the chest.
  • First and Second Stages of the Brachial Plexus: lie in the lower part of the posterior triangle
  • They lie between scalenus anterior and scalenus medius (the neurovascular plane of the neck).

• Arteries

  • Third part of subclavian artery.
  • Suprascapular artery.
  • Transverse cervical artery.
  • Occipital artery.

• Veins

  • Subclavian vein.
  • External jugular vein.

• Muscle

  • Inferior belly of omohyoid muscle.

Sternocleidomastoid

• Origin by 2 heads:
  • Sternal head: from the front of the manubrium sterni.
  • Clavicular head: from the medial 1/3 of the clavicle.
• Insertion:
  • Lateral surface of the mastoid process.
  • The lateral 1/2 of the superior nuchal line.
• Nerve Supply: Spinal accessory nerve.

Sternocleidomastoid Actions

• Acting alone: The Sternocleidomastoid muscle tilts the head to the same side in which it is contracted, turning the face to the opposite side.
• Acting together: The Sternocleidomastoid muscle flexes the neck and assists in inspiration through the elevation of the clavicle.
• Spasm of the muscle (torticollis) is called spastic neck (due to cold, inflammation), in which the ear is tilted toward the contracted muscle, and the chin faces the opposite direction.